1. Technical Field
The present disclosure relates generally to a fuel cell system using aqueous solution fuels, and more particularly to a fuel cell system and a method that corrects concentration sensing values by estimating temperature according to the load on a stack.
2. Discussion of Related Art
A fuel cell is a power generation system that generates electric energy by a balanced electrochemical reaction, for example, between oxygen in the air and hydrogen contained in hydrocarbon compounds and their derivatives such as methanol, ethanol, and natural gas, etc.
Fuel cells can be categorized into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells, alkaline fuel cell, and the like, according to type of electrolyte used therein. These types of fuel cells operate on the basic same principle, but differ in view of types of fuels used, operating temperatures, catalysts, electrolytes, and the like.
Compared with other types of fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) has advantages including high output, low operating temperature, and rapid starting and response characteristics, and is widely applicable as a transportable power source, such as for a portable electronic device, or as a mobile power source, such as a power source for automobile, as well as a distributed power source, such as a stationary power plant used in a house, a public building, and the like. The polymer electrolyte membrane fuel cell generates electricity by using gaseous fuel, typically, molecular hydrogen.
A direct methanol fuel cell (DMFC) is similar to the polymer electrolyte membrane fuel cell, but is capable of directly using liquid methanol as a fuel, generally, as an aqueous methanol solution. The direct methanol fuel cell is advantageous in view of miniaturization since it does not need a reformer to form hydrogen fuel in contrast to the polymer electrolyte membrane fuel cell.
A direct methanol fuel cell includes a stack, a fuel tank, and a fuel pump, for example. The stack generates electric energy by electrochemically reacting a hydrogen-containing fuel and an oxidant, such as oxygen and air, etc. Such a stack generally has a structure in which several or several tens of single fuel cells comprising a membrane electrode assembly (MEA) and a separator are stacked. Herein, the membrane electrode assembly has a structure in which an anode electrode (referred to as “fuel electrode” or “oxidation electrode”) and a cathode electrode (referred to as “air electrode” or “reduction electrode”) are adhered to each other, with a polymer electrolyte membrane disposed therebetween.
A fuel cell such as the direct methanol fuel cell in which the fuel to the stack is supplied as an aqueous solution exhibits great differences in operational efficiency according to the mole concentration of the fuel supplied to the anode and cathode electrodes. For example, if the mole concentration of the fuel supplied to the anode electrode is high, the amount of fuel transferred from the anode side to the cathode side increases due to limitations of currently available polymer electrolyte membranes. Therefore, a counter-electromotive force occurs due to the reacted fuel at the cathode electrode side thereby reducing output. Accordingly, the fuel cell stack has optimal operation efficiency at a predetermined fuel concentration according to its constitution and characteristics. As a result, a need exists for a method of properly controlling the mole concentration of fuel so as to achieve the stable operation of the direct methanol fuel cell system.
Therefore, the direct methanol fuel cell system and the like may include a means for measuring a concentration of solution stored in components such as a stack, a fuel tank, and a mixing tank, and/or a concentration of solution flowing in conduits between such components. In a fuel cell, a driving state of a fuel cell system can be estimated by measuring a concentration of fuel aqueous solution, etc., and each component of the fuel cell system is controlled according to the estimated result so that the operation efficiency of the fuel cell can be improved.
In order to satisfy the requirements described above, commonly used concentration sensors include polymer adsorption-type concentration sensors, ultrasonic-type concentration sensors comprising an ultrasonic generator and a detector, and resistance measurement-type concentration sensor, etc.
However, in currently widely employed concentration sensors, there are considerable deviations in the sensed values depending on the temperature of solution. Therefore, the temperature of solution at the sensed time point should be known in order to obtain accurate concentration. To this end, a dilution tank is provided with separate temperature sensors, which, however, increases manufacturing cost and volume.